This invention relates to a process for the preparation of urethanes. More particularly, it relates to a process for making urethanes which are suitable starting materials for the preparation of isocyanates by thermal dissociation.
The preparation of urethanes without the use of phosgene and subsequent thermal dissociation thereof into the corresponding isocyanates is a valuable alternative to the preparation of isocyanates by phosgenation of the corresponding primary amines of the prior art. According to U.S. Pat. Nos. 2,409,712 and 2,806,051, this phosgene-free urethane synthesis takes place by reacting urea with primary amines and alcohols at a temperature greater than 100.degree. C. It has been determined, however, that maximum yields are obtained when this synthesis is carried out at temperatures of 140.degree. to 200.degree. C. Therefore, when alcohols which boil at temperatures below the stated range are used in synthesizing urethanes, it is essential to carry out the process under pressure if maximum yields are desired. However, working under pressure requires an increased amount of apparatus and has the further disadvantage that the ammonia gas evolved during the reaction may not escape immediately after formation. The ammonia gas would dissolve in the reaction product and bring the reaction to a premature end, thereby causing poorer yields to be obtained. Consequently, attempts have been made to carry out the synthesis using comparatively high boiling alcohols so that it would be unnecessary to carry out the process under pressure. However, use of comparatively high boiling alcohols, e.g. alcohols, the boiling point of which is close to the boiling point of the isocyanate corresponding to the urethane, has the disadvantage that dissociation products (i.e., isocyanate and alcohols) are obtained from the subsequent thermal dissociation. These dissociation products are difficult to separate by distillation due to their similar boiling points. In the preparation of isocyanates by the thermal dissociation of urethanes, an immediate and effective separation of the dissociation products is critical in order to avoid recombination of the dissociation product.